Many devices are designed and intended to be used in an environment of very low, or even cryogenic temperatures, where they produce an electrical signal which must be carried into an area of higher temperature before the signal is utilized, tested, or transmitted. Quite often the lower temperature device is contained in a vacuum dewar vessel to achieve a high degree of thermal isolation, in order to eliminate the convective heat transfer loss that would otherwise occur. In using such a device, several challenges need to be overcome in constructing a signal conduit apparatus for carrying the signal from the cold device in the dewar to the warmer, "outside world" area. These challenges include preserving the integrity of the vacuum within the dewar, reducing the conductive heat transfer loss such as keeping heat from passing through the signal conduit to the device, and keeping signal loss to a minimum.
Prior art apparatus used for carrying such signals have encountered problems in meeting these challenges. Prior art low-signal-loss RF interconnection techniques typically rely upon traditional type coaxial cable. Such coaxial cables do provide low signal loss and maintain relatively good signal integrity, but are made of materials that cannot be successfully used in applications that require low out-gassing and long life, such as encountered in a long term vacuum environment. This is because the presence of organic dielectric materials and entrapped gasses within the coaxial cable structure leads to virtual leaks within the vacuum vessel. Such leaks introduce gases into the vacuum environment that are not readily absorbable via traditional gettering techniques, and thereby preclude the successful use of coaxial cable in long-life vacuum applications. Additionally, the presence of entrapped gasses caused by the basic structure of metal cladding or braiding over the dielectric materials, can cause vacuum failure leading to system level failures.
Coaxial cables also incur high thermal conduction losses that contribute unacceptable levels of parasitic heat loss to a system. Coaxial cable and other prior art apparatus are also generally bulky, and are often complex due to the increased number of parts needed to complete the apparatus and connect the signal conduit between the device and the "outside world." For example, coaxial cables require that there be some sort of interconnect hardware at each end, involving threaded connector backshells and housings that are an additional source of entrapped gas, and can cause vacuum failure over the life of the product as the gas is released.
Another approach is the utilization of bulk materials to provide thermal isolation and interconnection. This is primarily the method described in U.S. Pat. No. 4,739,633 ("Room Temperature to Cryogenic Electrical Interface") and U.S. Pat. No. 4,498,046 ("Room Temperature Cryogenic Test Interface"). The disadvantages of the approach described and taught by these patents include structural and fragility limitations involved with the handling of brittle interconnection material, and the rather substantial parasitic heat load occasioned by allowing the same material used to support the circuit to be cooled, to have an interface at room temperature. Additionally, it is difficult, if not impossible, to create an adequate hermetic seal around the area where the apparatus and its interconnection penetrate the dewar vessel.
Successful vacuum packaging requires that any materials used in the construction that are exposed to the vacuum must be sufficiently leak-tight, and have low outgassing properties so as not to cause an internal gas pressure in excess of approximately 10.sup.-4 Torr to develop over the desired lifetime of the product.
In order to minimize the cooling capacity requirement, physical size, and the total power consumption of the unit, the method of constructing the apparatus for interconnecting and physically supporting the device should maximize its thermal impedance. Accordingly, the present invention provides a frequency matched signal conduit apparatus comprising a micro-strip feed fabricated onto a material consistent with long vacuum life applications, such as ceramic or other crystalline materials, a vacuum vessel signal penetration member electrically connected to the micro-strip feed, and, in the preferred embodiments, a signal interconnect comprising thermally resistive, electrically conductive material that provides high thermal isolation and low signal loss, for electrically connecting the micro-strip feed network to the device to be cooled.
The various elements of the apparatus are preferably impedance matched, and the micro-strip feed provided with an impedance matching or conversion portion for matching the impedance of the device to be cooled with the rest of the system, in order to further enhance the thermal isolation properties of differently designed impedance systems available from smaller cross-sectional interconnection components. While such impedance matching is not necessary from an operational standpoint, such impedance matching is preferred. For further information concerning microstrip impedance matching, reference is made to Chapter 5 "Impedance Transformation and Matching", Pages 203-258 of Foundations For Microwave Engineering by Robert E. Collins, published by McGraw-Hill, Inc. of New York, N.Y., Copyright 1966, Library of Congress Catalog Card Number 65-21572.
One advantage that may be achieved by embodiments of the present invention in addition to minimizing heat transfer losses is an increase in vacuum life by the elimination of potential virtual leaks and outgassing from organic compounds. The utilization of a crystalline or ceramic support structure and other inorganic materials is consistent with long-life vacuum dewar applications.
Another advantage that may be achieved by embodiments of the present invention is the elimination of threaded fasteners normally found in traditional coaxial applications. Elimination of threaded fasteners and connector backshells further removes the likelihood of virtual leaks from trapped gasses.